The present invention relates to an antenna device operable to receive an electric wave of digital radio broadcasting.
A digital radio receiver which is operable to receive a satellite wave or ground wave and allows a person to hear digital radio broadcasting has been developed, and has been put to practical use in United States. The digital radio receiver can be mounted on a mobile station, such as a vehicle, or can be installed inside a building to receive an electric wave the frequency band of which is about 2.3 GHz and to allow a person to hear the radio broadcasting. Since the frequency of the received electric wave is about 2.3 GHz, a received wavelength (resonant wavelength) λ is about 128.3 mm. In addition, once the satellite wave is received at the earth station, the frequency thereof is shifted a little and is retransmitted as a linearly polarized wave that is the ground wave.
Since an electric wave having a frequency band of about 2.3 GHz is used in the digital radio broadcasting, it is preferable that an antenna device receiving the electric wave is installed outdoors. Therefore, when the digital radio receiver is mounted on a vehicle or the like, it is provided, for example, in the outside of the vehicle such as a roof thereof in many cases. On the other hand, when the digital radio receiver is used inside a building, the antenna device is installed in a roof, a balcony or the like in many cases.
Here, the directional sensitivity of this type of antenna device is omni-directional. However, in order to ensure the best sensitivity, the antenna device is required to be constructed such that the wave angle of the antenna device is adjusted to avoid an electric wave shielding object. The related-art antenna device for home use which has an angle adjustment function is disclosed in, for example, Japanese Patent Publication No. 2004-289514A. The related-art antenna device disclosed in Japanese Patent Publication No. 2004-289514A includes a base portion, and an antenna portion the angle of which is adjustable with respect to the base portion via a hinge portion. The hinge portion has a hinge base, two hinge bushes, two coil springs, two washer, and two screws. The hinge base is formed on an end of a ceiling surface of the base case of the base portion, and has two tapered shafts protruding from both ends of the hinge base, respectively. Each of the hinge bushes has a tapered inner peripheral surface and is fitted into each of the shafts of the hinge base. The coil spring is fitted into each of the shafts on which the hinge bush is mounted. A screw is fastened via the washer to an end surface of each of the shafts on which the hinge bush and the coil spring are mounted. The biasing force of the spring generates a frictional force between the outer peripheral surface of the shaft of the hinge base and the inner peripheral surface of the hinge bush when the hinge bush rotates. The antenna portion is joined to the hinge bush and is capable of being opened and closed at an arbitrary angle relative to the base portion. The frictional force between the shaft of the hinge base and the hinge bush causes the antenna device to be fixed at a constant angle without rotation when an external force is not applied thereto. On the other hand, when an external force larger than the frictional force between the shaft of the hinge base and the hinge bush is applied to the antenna portion, the antenna portion rotates, and can be adjusted to an arbitrary angle. The angle adjustment is performed as follows, for example. A user manually adjusts the angle of the antenna portion such that the best level of a broadcasting signal or the best sound quality of audio information received by a digital radio receiver is maintained, while confirming the level or the sound quality.
As described above, in the related-art antenna device including the example described in Japanese Patent Publication No. 2004-289514A, the number of the hinge bush, the coil spring, the washer, and the screw is two, respectively. For this reason, it is disadvantageous in decreasing the size and the weight of the antenna device. Also, since the cost of parts is high and since a number of manufacturing processes are required, it is disadvantageous the decreasing the cost thereof.
A satellite wave, especially a circularly polarized wave is used for digital radio broadcasting. As disclosed in Japanese Patent Publication No. 2005-20644A, an related-art antenna device including an antenna module corresponding to the circularly polarized wave has a ground plate, an antenna plate disposed to face the ground plate at a distance therefrom, an antenna probe disposed between the ground plate and the antenna plate, and a probe holder made of resin for holding the antenna probe. The probe holder is formed with a groove portion in which the antenna probe is to be disposed. A member made of an insulating material, such as a tape, for electrically insulating the antenna probe and the antenna plate is attached onto the ceiling surface of the probe holder in which the antenna probe is to be disposed.
For the purpose of realizing miniaturization, lightness, reliability, and cost reduction of the antenna device including the related-art antenna device disclosed Japanese Patent Publication No. 2005-20644A, it is effective to realize miniaturization, lightness, reliability, and cost reduction of the antenna module. Specifically, it is considered that, for example, the ground plate and the antenna plate is miniaturized. However, in a case where the ground plate and the antenna plate are miniaturized, there is a possibility that an electrostatic capacitance of the antenna module, which is required for the antenna device to exhibit a sufficient gain, may decrease.
An antenna device used for a GPS (global positioning system) using a satellite wave similarly to a digital radio broadcasting system, particularly an antenna device suitable for being installed on a roof of a vehicle is disclosed in Japanese Patent Publication No. 2005-109688A. The related-art antenna device disclosed in Japanese Patent Publication No. 2005-109688A includes a top cover, a bottom cover, an antenna module, a packing member, and a signal line. Since the packing member is disposed in a connecting portion between the bottom plate and the top plate to ensure adherence therebetween to perform a water-proof function, it is also called as a water-proof packing. The packing member has a generally angular-shaped plate part including a wide angular-shaped frame part and a bush part covering the periphery of the signal line in the position of a cut-out portion formed in the top cover. Since the frame part and the bush part are integrally formed with each other, the number of parts and manufacturing processes is small compared to a case in which the both members are separately formed. The water-proof function is realized as follows. A water-proof rib having an angular and frame shape is formed inside the top cover to correspond to the angular-shaped frame part of the plate part of the packing member. The packing member and the bottom plate are fastened to the top cover with a plurality of screws. The fastening pressure at this time causes the leading end of the water-proof rib of the top cover to be pressed against the angular-shaped frame part of the packing member, thereby realizing the water-proof function.
As described above, in the antenna device including the related-art antenna device disclosed in Japanese Patent Publication No. 2005-109688A, when the fastening pressures by the plurality of screws are different from one another or the fastening pressures by the screws are too large, there is a possibility that a gap may be formed between the water-proof rib formed in the top cover and the packing member. As a result, there is a possibility that the water-proof property realized by the packing member may deteriorate.